MIPS: KVM: Decode RDHWR more strictly

When KVM emulates the RDHWR instruction, decode the instruction more
strictly. The rs field (bits 25:21) should be zero, as should bits 10:9.
Bits 8:6 is the register select field in MIPSr6, so we aren't strict
about those bits (no other operations should use that encoding space).
Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim KrÄmář <rkrcmar@redhat.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Cc: kvm@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/mips/kvm/emulate.c

@@ -2357,7 +2357,9 @@ enum emulation_result kvm_mips_handle_ri(u32 cause, u32 *opc,
 	}
 
 	if (inst.r_format.opcode == spec3_op &&
-	    inst.r_format.func == rdhwr_op) {
+	    inst.r_format.func == rdhwr_op &&
+	    inst.r_format.rs == 0 &&
+	    (inst.r_format.re >> 3) == 0) {
 		int usermode = !KVM_GUEST_KERNEL_MODE(vcpu);
 		int rd = inst.r_format.rd;
 		int rt = inst.r_format.rt;